There have been developed various radiographic-image capturing devices such as a so-called direct-type radiographic-image capturing device which generates charges at a detection element according to the irradiated amount of radiation such as x-rays to convert them into electrical signals, and a so-called indirect-type radiographic-image capturing device which converts irradiated radiation into electromagnetic waves having other wavelength such as a visible light with a scintillator or the like, after which, according to the amount of energy of the converted and irradiated electromagnetic waves, generates charges at a photoelectric conversion element such as a photodiode, and converts them into electric signals. In the present invention, the detection element incorporated in the direct-type radiographic-image capturing device, and the photoelectric conversion element incorporated in the indirect-type radiographic-image capturing device, are collectively referred to as a radiation detection element.
The radiographic-image capturing device of this type has been known as an FPD (flat panel detector), and heretofore has been integrally formed with a supporting table (or a Bucky device) (refer, for example, to Patent Document 1), but, in recent years, a portable radiographic-image capturing device, in which the radiation detection element and the like are stored in a housing, was developed and has been practically used (refer, for example, to Patent Documents 2 and 3).
In such a radiographic-image capturing device, a plurality of radiation detection elements are arranged two-dimensionally (in a matrix state) to form a detecting unit, and, in that case, the number of radiation detection elements (that is, the number of pixels) usually amount to several millions to several tens millions or more pixels. For that reason, if image data which were read out from each radiation detection element are transferred to an external device without compression, the transfer time becomes longer. Further, in a portable radiographic-image capturing device incorporating a battery, if the transfer time of image data becomes longer, electric power consumed during the transfer becomes larger, resulting in exhaustion of the battery.
Then, as it is described in, for example, Patent Documents 4 and 5, the read out image data are usually compressed by a data compression method such as reversible compression (also referred to as lossless compression) and irreversible compression (also referred to as lossy compression), to be transferred to an external device such as a console and a server.
It is assumed that, in the case where, for example, the radiographic-image capturing device, in which an image of a part of the body of the patient as a subject, such as the head, the chest, and hands and feet, is captured, and the obtained radiographic image is used for diagnosis or the like as a medical image, is used as a medical image capturing device, as an image data compression method, in general, it is preferable to adopt the reversible compression method in which compression is carried out so that image data before compressed perfectly agree with restored image data rather than the irreversible compression method in which a part of information of image data is lost by compression.